Electronic keying circuit with selectable sustain characteristics

ABSTRACT

Post-keyswitch-release and percussion sustain keying circuits in which a variety of player selectable sustain control signals applied to a sustain control terminal coupled to a time constant circuit through a diode gate produce differing modes of sustain and in some modes player settable sustain intervals. In addition, several octave groups of percussion sustain keying circuits having the same timing circuit component values have scaled sustain time constants by varying the sustain control signal applied to each octave group.

United States Patent 1 1 i 11 1 3,924,505

Schrecongost [45] Dec. 9, 1975 [54] ELECTRONIC KEYING CIRCUIT WITH 3,637,915 1/1972 l-lirano 84/1.13 SELECTABLE SUSTAIN 3,665,091 5/1972 l-lirano 84/1.13 CHARACTERISTICS 3,715,445 2/1973 Kmepkamp 84/126 X 3,746,775 7/1973 Uchiyama 84/126 [75] Inventor: Ray B. Schrecongost, Park Ridge,

Ill. Primary ExaminerL. T. HiX 73 Assi nee: Hamm nd C C Assistant ExaminerU. Weldon 1 g In. 0 orpora hlcago Attorney, Agent, or Firm-l ,owell C. Bergstedt 22 Filed: Nov. 14, 1973 21 Appl. No.2 415,774

[57] ABSTRACT Post-keyswitch-release and percussion sustain keying circuits in which a variety of player selectable sustain 52 US. Cl. 84/1.01- 84/l.13' 84/1 26 comm Signals applied to a Sustain terminal [51] Int C| 2 G10H 1/00. G10H3/00. GIOH U02 coupled to a time constant circuit through a diode [58] Field of Search 84/1.01 1.26 1.13 1.04 gate Pmduce differing modes 0f Sustain and in some 84/117 119 DIG 8 DIG 23 modes player settable sustain intervalsv In addition, several octave groups of percussion sustain keying cir- [56] References Cited cuits having the same timing circuit component values have scaled sustain time constants by varying the sus- UNITED STATES PATENTS tain control signal applied to each octave group. 3,112,353 11/1963 Campbell 84/l.l3 X 3,591,702 7/1971 Adachi 84/].13 x 8 Chums, 10 Drawmg Flgures r 4; 3/ DUTY CYCLE CONTROL 1 'T I I I 23 4/C 1 FREE RUNNING 92 1 I MULTIVIBRATOR 8001 1 SELECTOR 1 I an: SWITCH 1 J TONE SIGNAL ,14v a0.4\. GENERATOR I FREE RUNNING :l 90 Ffi ji 424 I MULTIVIBRATOR d I DUTY CYCLE CONTR l SWITCH FREE RUNNING I MULTIVIBRAT U.S. Patent Dec. 9 1975 Sheet 1 of 2 3,924,505

FIG. I

I TONE sIGNAI I l I 3 GENERATOR I DUTY CYCLE CONTROL I I l 4/A I FREE RuNN N #92 I KEYER I G 4/ MULTIVIBRATOR N 28v I ma Iv-"IF 56 80C SW|TCH I TONE SIGNAL K32 I-mv 80A I GENERAToR I 'FREEl RUNNING 90 I ,1. h F-42A ULT VIBRATOR I v93 l O I l KEYEIKZEQQC 80B| I I DUTY CYCLE CONTROL 53 56 I 97 I 54 T T T T T l fi l ToNE SIGNAL GENERATOR I *1 DUTY CYCLE CONTROL I FREE RUNNING #94 I I MULTIVIBRATOR I/ 1 1-28, fia/E SELECTOR I 7/ 4V 81c SWITCH I FREE RUNNING I I ToNE'sIGNAL I MULTIVIBRATOR I GENERATOR I 95/ 8M 44 I DUTY CYCLE CONTROL 44c l FIG.3 00

FUNCTION GENERATOR A PERCUSSION KEYERS 6 SECOND OCTAVE PERCUSSION KEYERS FREE RUNNING MULTIVIBRATOR FUNCTION -GENERATOR C FUNCTION GENERATOR D PERCUSSION KEYERS FUNCTION GENERATOR E FIFTH OCTAVE PE RCUSSION KEYERS U.S. Patent Dec. 9 1975 Sheet 2 of2 3,924,505

FIG.2E

FIG.2A

TIME

TIME

KA KR Fl G. 2 F

KA R

O M 2 6503 2206 wzimx TIME TIME

m J M FIG. 28

KR TIME ELECTRONIC KEYING CIRCUIT WITI-I SELECTABLE SUSTAIN CHARACTERISTICS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates in general to electronic keying circuits for musical tone signals and in particular to post-key-release sustain keying circuits and percussion sustain keying circuits.

2. Description of the Prior Art It is well known in the art of electrical musical instruments to apply a keying voltage to an electronic keyer through a particular time constant circuit to produce a keying voltage waveform which has either a post-keyrelease sustain characteristic or a percussion sustain characteristic. To produce a post-key-release sustain characteristic, the time constant circuit typically icomprises a capacitor and a resistor separately connected between the keying signal input lead of the keyer and ground reference-the capacitor being charged upon operation of the key switch and, after release of the switch, being discharged through the resistor. The R-C time constant of the discharge path is selected to produce a desired sustain interval during which the keyer is gradually turned off by the decreasing voltage on the capacitor.

A percussion sustain characteristic is usually produced by employing a time constant circuit in which a capacitor is connected between the keyswitch and the keying signal input lead of the keyer and a resistor" is connected between the input lead and ground reference. Upon operation of the keyswitch, the keying voltage is initially applied to the keyer; but as the capacitor charges, the voltage on the keying signal input lead decreases to gradually turn off the keyer even though the keyswitch remains operated. Again the R-C time constant is selected to produce a desired sustain interval. 1 g

It is also well known to provide player-operated controls for switching different resistors with differing values in and out of the time constant networks to vary the sustain intervals, but this'approach provides limitedplayer control over sustain times since it is uneconomical to provide a large number of different resistors and switches in each time constant circuit of a particular instrument.

Another prior art system employs a time constant circuit which provides multiple stage decay of keying voltages with different decay rates or waveform slopes during each stage. Multiple stage decay is accomplished by employing a second discharge path, comprising a diode and resistor, between the keying signal input of the keyer and a voltage intermediate ground reference and the keying voltage. This second path is operative only until the capacitor discharges or charges, as the case may be, to the intermediate voltage. The principal value of a time constant circuit with multiple stage decay is realized when a player desires both a long sustain time on tones which are not promptly rekeyed and distinguishably separate tones during rapid, repetitive playing of a single key. Thisis. accomplished by utilizing a relatively small value resis-.

tor in the second discharge path and a relatively large value resistor in the first discharge path, such that the initial stage of decay has a short time constant so that the keying voltagedrops rapidly, whilethe second stage has a long time constant so that the keying voltage 2 drops slowly. The rapid delay of keying voltage immediately after key release produces a rapid initial decrease in tone volume such that prompt reactuation of the key will produce a distinct increase in tone volume.

However, to provide different selectable sustain intervals, the prior art multiple slope circuit also relies on multiplying the number of discharge resistors in each time constant circuit with external switches controlling the inclusion or exclusion of particular resistors. Thus, the number of player-selectable sustain intervals is restricted by the cost of the extra components required in each time constant circuit.

Hirano U.S. Pat. No. 3,665,091 discloses a sustain keyer circuit in which control of the sustain time is achieved by employing, in the time constant circuit, a field effect transistor (FET) switching element with its control electrode coupled to a source of chopped voltage having a variable duty cycleQThe duty cycle of the chopped voltage determines the percentage of time that the PET is in its fully conducting state which, in turn, controls the effective sustain time constant of the circuit. However, the Hirano circuit does not provide multiple stage decay type of operation; and thus, when set for long sustain times, repeated fast actuations of the same key will not produce distinctly separate tones.

SUMMARY OF THE INVENTION The present invention is principally directed to providing a sustain keying circuit which features both sustain interval control and multiple stage decay. The invention is useful in post-key-release sustain circuits and percussion sustain circuits. A second aspect of the invention involves the provision of a sustain keying circuit which can be operated in a plurality of modes under the control of the player, with sustain time control available in certain of the modes. A third aspect of this invention involves its use in a musical instrument in which different sets of notes require different sustain times to sound authentic, such as in the piano voicing of an electronic organ or in an electronic piano. Instead of providing time constant circuit elements with scaled values in keyers for notes of different octaves, this invention provides for the use of the same values for circuit elements in all octaves with scaling of sustain times accomplished by employing sustain control signals with different parameters.

All sustain keying circuits, including both post-keyrelease sustain and percussion sustain circuits, are characterized by a resistor and a capacitor effectively coupled in a series timing circuit during the sustain interval of a keying cycle with the junction of the resistor and capacitor coupled to the keying signal lead of the keyer. During this sustain interval, the capacitor is either charging or discharging depending upon the type of time constant circuit and its construction. In accordance with this invention, a second resistor and a diode are serially connected between the capacitor-firstresistor junction and a sustain control terminal, and circuit means are provided toapply a sustain control signal to the sustain control terminal. According to a particular aspect of this invention, a sustain control signal of a type comprising a chopped voltage varying with a preselectable duty cycle between ground reference voltage and a voltage intermediate ground and the keying voltage applied to the time constant circuit is employed. Such a sustain control signal produces a two-stage decay waveform-thefirst stage comprising the time interval during which the voltage onjthe keying signal leaddecays to the interrnediate voltage and thesecond stage. comprising the interval during. which the voltage decays-between theintermediate voltage and ground reference. During both stages of.decay,.the

chopped voltage alters the time constant of the overall R-C timing circuit from the value it would have under the condition of application of a steady intermediate voltage as in the priorart circuit discussed above. Al'- tering the duty cycle of the chopped voltage,:i.e'., the

percentage' of time the signal is at the intermediate voltagelevel, produces'differing effective R-C'time constants and thus provides a ready controlov'er the sustain time ofthe keying circuit. In practice, the value of the second resistor is. chosen to be considerably smaller than the first resistor such that the first stage of decay willhave a relatively short time constant for repeatability purposes and the second stage will have a longtime constant for producing a lingering tone. The

chopped voltagesignal has the largest effect on the time constant of the second stage during which the control of sustain time is most useful.

According to another aspect of this invention, the

player can select-one of several types of sustain control signals which produce different modes of operation of the keying circuit. In addition to the chopped voltage signal varying between ground and an intermediate voltage denoted Type (A), one of the following types of sustain control signal may be selected:

Type (B): ground reference,

Type (C'): the intermediate voltage,

Type (D): a chopped voltage signal varying with a preselectable duty cycle between ground reference and the keying voltage,

Type (E): the keying voltage.

Sustain control signal Type (B).produces a single stage decay with'theshortesttime constant and, in effect, no sustain because the lower 'value second resistor produesa very short time constant. Type (C) produces a two-stage decay with short and long time constants,v

respectively, in the first and second stages due to the stage, long decay, because-the second resistor is, in

effect, absent from the time constant circuit.

Fromthe 'above description it'shouldbe apparent that this invention provideshighly advantageous player control over the mode and length of sustain such that individual preferences'of differentplayers are satisfied and the" sustain keying can be tailored to a particular gomposition being played on theinstrument. This is accomplished without the requirement of multiple tim- ,ing circuit components-ineach time constant circuit by providing the various sustain control signals in common to a large number of time constant circuits.-

BRIEF DESCRiPTION OF DRAWINGS 1 is a partly .blockandpartly circuit schematic diagram illustrating a preferred embodiment of this f 5. 1= plu ra lity;ofkeyers whenthis invention is deployed in a waveformsillustrating the'various 65 FIG. 3 is a block diagram. of a prepferred-embodi-n invention. FIGS..2A21-I are modes of operation of, the circuitry of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 shows a keying bus. 10, a rank of keyswitches 20, a plurality of tone signal generators 31 to 34, a

plurality of keyers 41 to 44, a plurality of time constant circuits 51, 52, 61, and 62, a pair of sustain control terminals and 71, and a pair of sustain control signal circuits and 81.

Keying bus 10 supplies a negative 28-volt keying voltage to each of keyswitches 21, 22, and 23 in rank 20. Together these elements constitute a keying voltage circuit. In an electrical musical instrument, such as an electronic organ or electronic'piano, a number of keyswitches associated with at least several octaves of musical notes would typically be provided, and only a few such keyswitches are shown here to illustrate this invention. When actuated, keyswitch 21 applies the keying voltage to time constant circuits 51 and 61. Similarly, keyswitch 22 applies the keying voltage to time constant circuits 52 and-62. Keyswitch 23 is shown to illustrate that additional keyswitches would typically be employed with similar associated circuitry.

Time constant circuits 51 and 52 are identical in structure, and they function to control the decay of keying signal voltage on keying signal input leads 41B and 42B of keyers 41 and 42,respectively, after release of associated keyswitches 21 and 22. Keyers 41 and 42 receive respective tone signals from tone signal generators 31 and 32 on input leads 41A and 42A and function to produce keyed tone signals on terminals 41C and 42C in response to keying signals on leads 41B and 42B. The amplitude of the keyed tone signal is proportional to the magnitude of the keying signal. Keyswitch 21, time constant circuit 51, and keyer 41 together comprise a keying circuit having a post-key-release sustain characteristic, and the same is true of the combination of keyswitch 22, time constant circuit 52, and keyer 42.

Time constant circuits 61 and 62 are identical in structure, and they function, when their associated keyswitches 21 and 22 are operated, to supply percussion sustain keying signals to input leads 43B and 44B from tone signaligenerators 33 and 34 to produce keyed tone signals on terminals 43C and 44C..The keyed tone signals have an amplitude proportional to the magnitude of the keying signal. Keyswitch 21, time constant circuitv 61, and keyer 42 comprise a keying circuit having a percussion sustain characteristic. The sameis trueof the combination of keyswitch 22, time constant circuit '62, and keyer 44.

Preferably keyers 41 to 44 comprise either the type of keyer disclosed in Schrecongost et al U.S. Pat. No. 3,636,231 or the type disclosed in Ring US. Pat. No. 3,534,144. As shown in the Schrecongost et al patent, a single time constant circuit is employed with a plurality of dual field effect transistor (FET) keyers, each of a multioctave instrument.

which receives a separate tone signal. Thus, although FIG. 1 herein shows only one keyer associated with each timeconstant circuit, it is to be'understood that eachtime constant circuit is generally common to a nal as the keyed tonesignal by combining a plurality of octavely related tone signals in.a weighted fashion.

Thus, it is to be understood that, when the present invention is deployed in a keying circuit as disclosed in the Ring patent, a plurality of tonegenerators feed each keyer. The various ways in which keyed tone signals are processed through other circuitry are well known and need not be described have since they form no part of the presentinvention. Preferably, the tone signal generators are square wave signal sources of the frequency divider typewhich are also well known in this art.

Time constant circuit 51 comprises a resistor 55 and a capacitor 56 connected between keying signal lead 41B and ground reference, and a second resistor 54 and a diode 53 connected in series between lead 41B and a sustain control terminal 70. Time constant circuit 52 has similar components connected in a similar fashion. A selector switch 90 selectively applies to sustain control terminal 70 one of the following types of signals:

Type A: a chopped voltage signal varying between ground reference and a negative 14 volts produced on lead 80A by free-running multivibrator 93.

Type B: ground reference on lead 80B.

Type C: negative 14 volts on lead 80C.

Type D: a chopped voltage signal varying between ground reference and the negative 28-v0It keying voltage produced on lead 800 by free-running multivibrator 92.

Type E: negative 28-volts on lead 80E.

Duty cycle controls 96 and 97 select the on and off periods of multivibrators 92 and 93, respectively, to vary the sustain interval in a manner which will be explained in detail below. Sustain control circuit 80 produces a low impedance at sustain terminal 70 for all sustain control signals applied thereat.

In a preferred embodiment, resistor 54 is selected to have resistance value much smaller than resistor 55, typically about two orders of magnitude smaller. Assume selector switch 90 connects lead 80B to sustain control terminal 70 so that sustain control signal Type B is applied thereat. The operation of the keying circuit under this signal condition on terminal 70 is as shown in FIG. 2A. When keyswitch 21 is actuated at time KA, the keying signal on lead 41B drops rapidly to -28 volts as capacitor 56 charges rapidly to the keying potential. (A small value attack control resistor, not shown, is usually employed to eliminate key click transients.) The keying signal remains at -28 volts until keyswitch 21 is released at time KR, whereupon capacitor 56 discharges rapidly via the path through diode 53 and resistor 54 to ground reference on sustain control terminal 70. The amplitude of the keyed tone signal also declines rapidly after key release and. in effect, is not sustained to any noticeable degree. Resistor 55 provides a parallel discharge path under this condition; but almost all of the discharge current flows through resistor 54, and the decay time constant of the keying signal is determined almost solely by the values of resistor 54 and capacitor 56.

Next assume switch 90 selects the -28 volts signal on lead 80E to be supplied to sustain control terminal 70.

FIG. 2B depicts the keying signal waveform under application of this sustain control signal Type. E. Upon key release at time KR, capacitor 56 discharges very slowly through resistor 55. Resistor 54 is effectively out of the circuit because diode 53 is rendered non-conducting by the -28 volts on terminal 70. The decay time constant is determined solely by the values of resistor 55 and capacitor 56. This condition produces a long sustain of the keyed tone signal after key release, but it can readily. be seen that a prompt reactuation of the keyswitch would not produce an audibly separate keyed tone.

FIGS. 2C and 2D show the keying signal waveform when a Type D sustain control signal is applied at terminal 70. Under such condition, the signal on terminal is a chopped voltage signal varying between -28 volts and ground reference as shown in dashed lines marked D. At point KR, when the keyswitch is released, the signal on terminal 70 is at -28 volts. Thus, diode 53 is non-conducting and the discharge rate of capacitor 56 is slow because only the discharge path through resistor 55 is effective. Then shortly thereafter the signal on terminal 70 drops to ground for an interval, and the keying signal decays rapidly due to rapid discharge of capacitor 56 through resistor 54. This alternative slow and fast decay continues until capacitor 56 is fully discharged. In FIG. 2C the duty cycle of the multivibrator is about 70 percent, and the effective time constant of the decaying keying signal is considerably shorter than that of FIG. 2B. In FIG. 2D the duty cycle is reduced to about 50 percent by varying duty cycle control 96, and correspondingly the effective time constant is shorter than that of FIG. 2C. It should be apparent that, by varying the duty cycle of multivibrator 92, the degree of sustain of the keyed tone after key release can be altered over a wide range.

When selector switch places l4 volts from lead 80C on sustain control terminal 70 (Type C sustain control signal), the keying signal waveform shown in FIG. 2E results. After key release at time KR, the keying signal decays rapidly to l 4 volts as capacitor 56 is discharged rapidly through resistor 54. When the signal on lead 413 reaches -14 volts, diodes 53 turns off, and capacitor 56 continues to discharge slowly through resistor 55. Thus a dual stage decay waveform is produced to provide the advantage of both long sustain and audibly distinct repetition of the same tone when the keyswitch is promptly reactuated after release.

Switching the Type A sustain control signal on lead 80A onto terminal 70 produces the keying signal waveform shown in FIG. 2F. The decay portion of the waveform in FIG. 2F, after key release at time KR, has a dual slope characteristic as did the waveform in FIG. 2E, but the effective decay time constant of the second stage of decay is altered by the chopped voltage signal. Immediately after key release, the keying signal on lead 418 drops rapidly to l4 volts because diode 53 is conducting during this entire interval and resistor 54 provides a rapid discharge path for capacitor 56. The alternating l4V and ground reference signal effects this stage of decay of the waveform only slightly. After the voltage on lead 418 drops below -14 volts, diode 53 conducts only when the sustain control signal waveform 80A, which is provided to sustain control terminal 70, is at ground reference. Thus the time constant of the second stage of decay is shortened by the intermittent operation of diode 53 during which capacitor 56 v discharges at a more rapid rate through resistor 54.

Varying the'duty cycle of multivibrator 93 produces a corresponding variation in the effective time constant of the'second stage of decay in a manner similar to that depicted in FIGS. 2C and 2D. From this it can readily be appreciated that three important advantages result: (I) Prompt initial decay of the keying signal waveform to enable fast repetitive keying to produce audibly separate keyed tones, (2) long overall decay for long sustain of the keyed tone signal if keying is not promptly repeated, and (3) continuously variable control of the sustain interval over a wide range without duplicating circuit components in each time constant circuit associated with each keyswitch.

In practice the frequency of multivibrators 92 and 93 would be designedto be above audible range to prevent the chopped voltage signals from adding audible background noise into the output of the instrument. It should be readily apparent, however, that the frequency of the chopped voltage signal does not change substantially the effective decay time constant at any duty cycle setting since it is the total on and off time which determines that effective time constant. The sustain control signal selected by selector switch 90 to be placed on sustain control terminal 70 is supplied in common to all of the post-key-release time constant circuits associated with a rank of key switches. By placing selector switch 90 and duty cycle controls 96 and 97 on a control panel of the instrument, the player has the capability to select the mode of sustain he desires and, in selected modes, the effective sustain interval.

The percussion sustain keying circuits in accordance with this invention will not be described. Referring again to FIG. 1, time constant circuit 61 has a capacitor 64 connected between keyswitch 21 and keying signal input lead 433, and a resistor 68 connected between lead 43B and ground reference. Diode 67 and resistor 66 are also connected in series between lead 433 and sustain control terminal 71. resistor 63 and diode 65 are connected, as shown, between opposite sides of capacitor 64 and ground reference. Diode 65A is connected at the point shown between capacitor 64 and lead 43B. Capacitor 69 is connected between lead 43B and ground reference. The percussion sustain control signal circuits 81 are identical to the post-key-release sustain control signal circuits 80. t Assume thecondition' that selector switch 91 has placed 28 volts Type E sustain control signal from lead 81E on sustain control terminal 71. When keyswitch 21 is actuated, the 28 volt keying voltage is applied to time constant circuit 61, and the voltage on keying signal input lead 43B drops promptly. to about 28 volts and capacitor 69 promptly charges to the keying voltage. (An attack control resistor, not shown, is usually provided between diode 65A and capacitor 69 to prevent key click transients.) With keyswitch 21 remaining in an actuated condition, capacitor 64 begins to charge via resistor 68 and the voltage on lead 438 begins to rise toward ground reference. With 28 volts on terminal 71, diode 67 is non-conducting, and thus resistor 66 does not participate in charging capacitor 64. Similar to time constant circuit 51, resistor 68 is selected to be about two orders of magnitude larger in value than resistor 66. Consequently, capacitor 64 charges very slowly due to, the long time constant, and the keyed tone signal has a long sustain interval. The decay envelope or waveform has an appearance very similar to the post-key-release decay waveform shown in FIG. 2B. However, if keyswitch 21 is released during the long sustain interval, capacitor 64 is discharged very quickly via resistor 63 and diode 65, and capacitor.

69, which has a small value, discharges through resistor 68 with a short time constant to prevent .key release transients. a

Without going through a detailed explanation, it should be apparent that the various types of sustain control signals which can be selected by switch 91 to be applied to sustain control terminal 7] determine the percussion sustain time constant during key actuation in a manner very similar to the way in which the same types of signals on terminal control the post-keyrelease sustain time constants. The only difference is that the timing capacitor in the percussion sustain circuit is charging during the sustain interval, while the timing capacitor in the post-key-release sustain circuit is discharging during the sustain interval. In both cases, the voltage on the keying signal input lead of the corresponding keyer is decaying with a decay waveform determined by the signal on a respective sustain control terminal. Comparing FIG. 2E and FIG. 2G is illustrative of this principle. The waveform in FIG. 2E represents the keying signal when l4 volts is applied to terminal 70, while the waveform in FIG. 2G represents the keying signal when l4 volts is applied to terminal 71. The decay time constants are the same until key release in FIG. 2G. The same is true for the comparison between FIG. 2F and FIG. 2H.

FIG: 211 represents the keying signal waveform with a chopped voltage varying between ground reference and l4 volts (Type E) applied to sustain control terminal 71. As shown, the decay portion of the waveform is very steep, between 28 volts and l4 volts, as capacitor 64 charges via resistor 66 and diode 67. Thereafter resistor 66 drops out of the charging circuit each time the potential on terminal 71 goes to l4 volts because diode 67 is non-conducting during that interval. The second stage of decay between l4 volts and ground reference proceeds at a faster rate than that shown in FIG. 2G because of the intermittent switching of resistor 66 in and out of the charging circuit for capacitor 64. Varying the duty cycle of the chopped voltage signal (i.e., multivibrator alters the effective decay time constant of the second stage of decay in a manner similar to that depicted in FIGS. 2C and 2D. From this discussion it can readily be appreciated the application of a chopped voltage waveform varying between ground reference and a voltage intermediate ground and the keying voltage with a selectable duty cycle provides a percussion sustain keying circuit with these advantages: (1) Prompt initial decay of the keying signal waveform to produce an authentic percussion strike tone, (2) a long overall decay interval if the keyswitch is kept actuated, and (3) continuously variable control of the sustain interval over a wide range without duplicating circuit components in each time constant circuit associated with each keyswitch.

To produce authentic piano-type percussion tones over a range of four or five octaves requires that the percussion-sustain time constant be shorter for the high note groups than the low note groups. A typical prior art practice is to group the percussion keying circuits by octaves and to use different timing circuit component values from group-to-group to achieve different sustain time constants. This would need to be done for the circuit of FIG. 1 if all' of the percussion time constant circuits for several octaves of notes are controlled by the same sustain control terminal. However, FIG. 3 illustratesan alternate approach in which a single multivibrator drives five separate function generators tors having differing ON times. All of the percussion keying circuits have identical timing circuit component values, and the scaled duty cycles of the function generators produce scaled sustain intervals from one active group to another.

From the above discussion, it should be apparent that this invention provides highly advantageous playercontrolled, multimode sustain keying circuits of both the post-key-release sustain and percussion sustain types. In particular, this invention provides dual slope sustain keying circuits with continuously variable sustain time constants without duplication of timing circuit components in the large number of time constant circuits employed in a typical electronic organ or piano. It should also be apparent that this invention is not limited to the specific embodiments disclosed in the drawings and discussed in detail above, since numerous modifications could be made without departing from the scope of this invention as claimed in the following claims.

I claim: 7

1. In an electrical musical instrument, a tone signal generator and a keying circuit coupled to said generator to produce a keyed tone signal having a selectably variable decay envelope, said keying circuit comprismg:

a keyer, including first and second input leads receiving a tone signal and a keying signal, respectively, operative to produce a keyed tone signal having an amplitude proportional to the magnitude of said keying signal;

a time constant circuit having a keying voltage input lead and keying signal output lead coupled to said second input lead; and

a keying voltage circuit for selectably applying a first voltage to said keying voltage input lead of said time constant circuit;

said time constant circuit including a first resistor and a capacitor coupled to said keying voltage input lead to form a series timing circuit during a terminal portion of an operating interval of said time constant circuit with the junction of said first resistor and said capacitor coupled to said keying signal output lead, a second resistor and a diode coupled in series between said junction and a sustain control terminal, and circuit means for applying to said terminal a sustain control signal comprising a chopped voltage signal varying with a preselectable duty cycle between ground reference and a second voltage intermediate ground reference and said first voltage; whereby a multiple stage voltage decay signal is produced on said second input lead of said keyer with the effective time constant of each stage varying with said preselectable duty cycle to produce said selectably variable decay envelope of said keyed tone signal.

2. Apparatus as claimed in claim 1, wherein said keying voltage circuit comprises a DC. keying bus and a keyswitch coupled between said bus and said keying voltage input lead of said time constant circuit; and said time constant circuit is a post-keyswitch-release sustain circuit with said first resistor and said capacitor each connected between said keying signal output lead and ground reference such that said multiple stage voltage decay signal is produced during discharging of said capacitor after release of said keyswitch.

3. Apparatus as claimed in claim 1, wherein said keying voltage circuit comprises a DC. keying bus and a keyswitch coupled between said bus and said keying voltage input lead of said time constant circuit; and said time constant circuit is a percussion sustain circuit with said capacitor coupled between said keyswitch and said keying signal output lead of said keyer and said first resistor coupled between said keying signal output lead and ground reference such that said multiple stage voltage decay signal is produced during charging of said capacitor after initial operation of said keyswitch.

4. In an electrical musical instrument, a tone signal generator and a keying circuit coupled to said generator to produce a keyed tone signal having a selectably variable decay envelope, said keying circuit comprismg:

a keyer, including first and second input leads receiving a tone signal and a keying signal, respectively, operative to produce a keyed tone signal having an amplitude proportional to the magnitude of said keying signal; I v

a time constant circuit having a keying voltage input lead and a keying signal output lead coupled to said second input lead; and

a keying voltage circuit for selectably applying a first voltage to said keying voltage input lead of said time constant circuit;

said time constant circuit including a first resistor and a capacitor coupled to said keying voltage input lead to form a series timing circuit during a terminal portion of an operating interval of said time constant circuit with the junction of said first resistor and said capacitor coupled to said keying signal output lead, a second resistor and a diode coupled in series between said junction and a sustain control terminal, and circuit means for supplying to said sustain control terminal a selectable one of a plurality of sustain control signals comprising:

first circuit means for generating a ground reference voltage as a first sustain control signal to produce a single stage voltage decay signal having an effective time constant depending solely on the: values of said capacitor'and said first and second resistors;

second circuit means for generating a chopped voltage signal varying with a preselectable duty cycle between ground reference and a second voltage intermediate ground reference and said first voltage as a second sustain control signal to produce a multiple voltage decay signal with the effective time constant of each stage varying with said preselectable duty cycle;

third circuit means for generating a second voltage as a third sustain control signal to produce a multiple stage voltage decay signal having an effective time constant of each stage dependent upon the values of said capacitor, said first and second resistors and said second voltage;

fourth circuit means for generating a chopped voltage signal varying with a preselectable duty cycle between ground reference and said first voltage as a fourth sustain control signal to produce a single stage voltage decay signal having an effective time constant varying with said preselectable duty cycle;

fifth circuit means for generating said first voltage as a fifth sustain control signal to produce a single stage voltage decay signal having an effective time constant depending solely on the values of said capacitor and said first resistor; and

switching circuit means for selectably coupling one of said first through fifth circuit means to said sus- 1 1 tain control terminal.

5. Apparatus as claimed in claim4, wherein said keying voltage circuit comprises a DC. keying bus and a keyswitch coupled between said busand said keying voltage input lead of said time constant circuit; and said time constant circuit is a keyswitch-release sustain circuit with said first resistor and said capacitor each connected between said keying signal output lead and ground reference such that said voltage decay signal is produced during discharging of said capacitor after release of said keyswitch.

' 6. Apparatus as claimed in claim 4, wherein said keying voltage circuit comprises a DC. keying bus and a keyswitch coupled between said bus and said keying voltage input lead of said time constant circuit; and

time constant circuit is a percussion sustain circuit with said capacitor coupledbetween said keyswitch and said keying signal output lead of said keyer and said first resistor coupled between said keying signal output lead and ground reference such that said voltage decay signal is produced during charging of said capacitor after initial operation of said keyswitch.

7. In an electrical musical instrument, a plurality of sets of percussion keying circuits and a plurality of tone signal generators coupled to said keying circuits, each set of percussion keying circuits being associated with a set of sequential notes of the musical scale and each of said keying circuits comprising a keyer, including first and second input leads receiving a tone signal and a keying signal, respectively, operative to produce a keyed tone signal having an 12. amplitude proportional to the magnitude of said keying signal;

a time constant circuit coupled to said second input I lead; and

a keyswitch for applying a first voltage to said time constant circuit;

said time constant circuit including a capacitor coupled between said keyswitch and said second input lead of said keyer, a first resistor coupled between said second input lead and ground reference to provide a first charging path for said capacitor, and a second charging path coupled to said second input lead and including a gating element coupled to a sustain control terminal;

said capacitor and said first and second charging paths having respective values indentical in all of said keying circuits and each of said sets of keying circuits having a separate sustain control terminal;

circuit means for applying to said sustain control terminals a plurality of sustain control signals each comprising a chopped voltage signal having a differing preselectable duty cycle for operating said gating elements to produce differing sustain intervals for said sets of keying circuits.

8. Apparatus as claimed in claim 7, wherein said second charging path further comprises a second resis tor and a diode as said gating element coupled between said second input lead and said sustain control terminal. 

1. In an electrical musical instrument, a tone signal generator and a keying circuit coupled to said generator to produce a keyed tone signal having a selectably variable decay envelope, said keying circuit comprising: a keyer, including first and second input leads receiving a tone signal and a keying signal, respectively, operative to produce a keyed tone signal having an amplitude proportional to the magnitude of said keying signal; a time constant circuit having a keying voltage input lead and keying signal output lead coupled to said second input lead; and a keying voltage circuit for selectably applying a first voltage to said keying voltage input lead of said time constant circuit; said time constant circuit including a first resistor and a capacitor coupled to said keying voltage input lead to form a series timing circuit during a terminal portion of an operating interval of said time constant circuit with the junction of said first resistor and said capacitor coupled to said keying signal output lead, a second resistor and a diode coupled in series between said junction and a sustain control terminal, and circuit means for applying to said terminal a sustain control signal comprising a chopped voltage signal varying with a preselectable duty cycle between ground reference and a second voltage intermediate ground reference and said first voltage; whereby a multiple stage voltage decay signal is produced on said second input lead of said keyer with the effective time constant of each stage varying with said preselectable duty cycle to produce said selectably variable decay envelope of said keyed tone signal.
 2. Apparatus as claimed in claim 1, wherein said keying voltage circuit comprises a D.C. keying bus and a keyswitch coupled between said bus and said keying voltage input lead of said time constant circuit; and said time constant circuit is a post-keyswitch-release sustain circuit with said first resistor and said capacitor each connected between said keying signal output lead and ground reference such that said multiple stage voltage decay signal is produced during discharging of said capacitor after release of said keyswitch.
 3. Apparatus as claimed in claim 1, wherein said keying voltage circuit comprises a D.C. keying bus and a keyswitch coupled between said bus and said keying voltage input lead of said time constant circuit; and said time constant circuit is a percussion sustain circuit with said capacitor coupled between said keyswitch and said keying signal output lead of said keyer and said first resistor coupled between said keying signal output lead and ground reference such that said multiple stage voltage decay signal is produced during charging of said capacitor after initial operation of said keyswitch.
 4. In an electrical musical instrument, a tone signal generator and a keying circuit coupled to said generator to produce a keyed tone signal having a selectably variable decay envelope, said keying circuit comprising: a keyer, including first and second input leads receiving a tone signal and a keying signal, respectively, operative to produce a keyed tone signal having an amplitude proportional to the magnitude of said keying signal; a time constant circuit having a keying voltage input lead and a keying signal output lead coupled to said second input lead; and a keying voltage circuit for selectably applying a first voltage to said keying voltage input lead of said time constant circuit; said time constant circuit including a first resistor and a capacitor coupled to said keying voltage input lead to form a series timing circuit during a terminal portion of an operating interval of said time constant circuit with the junction of said first resistor and said capacitor coupled to said keying signal output lead, a second resistor and a diode coupled in series between said junction and a sustain control terminal, and circuit means for supplying to said sustain control terminal a selectable one of a plurality of sustain control signals comprising: first circuit means for generating a ground reference voltage as a first sustain control signal to produce a single stage voltage decay signal having an effective time constant depending solely on the values of said capacitor and said first and second resistors; second circuit means for generating a chopped voltage signal varying with a preselectable duty cycle between ground reference and a second voltage intermediate ground reference and said first voltage as a second sustain control signal to produce a multiple voltage decay signal with the effective time constant of each stage varying with said preselectable duty cycle; third circuit means for generating a second voltage as a third sustain control signal to produce a multiple stage voltage decay signal having an effective time constant of each stage dependent upon the values of said capacitor, said first and second resistors and said second voltage; fourth circuit means for generating a chopped voltage signal varying with a preselectable duty cycle between ground reference and said first voltage as a fourth sustain control signal to produce a single stage voltage decay signal having an effectiVe time constant varying with said preselectable duty cycle; fifth circuit means for generating said first voltage as a fifth sustain control signal to produce a single stage voltage decay signal having an effective time constant depending solely on the values of said capacitor and said first resistor; and switching circuit means for selectably coupling one of said first through fifth circuit means to said sustain control terminal.
 5. Apparatus as claimed in claim 4, wherein said keying voltage circuit comprises a D.C. keying bus and a keyswitch coupled between said bus and said keying voltage input lead of said time constant circuit; and said time constant circuit is a keyswitch-release sustain circuit with said first resistor and said capacitor each connected between said keying signal output lead and ground reference such that said voltage decay signal is produced during discharging of said capacitor after release of said keyswitch.
 6. Apparatus as claimed in claim 4, wherein said keying voltage circuit comprises a D.C. keying bus and a keyswitch coupled between said bus and said keying voltage input lead of said time constant circuit; and time constant circuit is a percussion sustain circuit with said capacitor coupled between said keyswitch and said keying signal output lead of said keyer and said first resistor coupled between said keying signal output lead and ground reference such that said voltage decay signal is produced during charging of said capacitor after initial operation of said keyswitch.
 7. In an electrical musical instrument, a plurality of sets of percussion keying circuits and a plurality of tone signal generators coupled to said keying circuits, each set of percussion keying circuits being associated with a set of sequential notes of the musical scale and each of said keying circuits comprising a keyer, including first and second input leads receiving a tone signal and a keying signal, respectively, operative to produce a keyed tone signal having an amplitude proportional to the magnitude of said keying signal; a time constant circuit coupled to said second input lead; and a keyswitch for applying a first voltage to said time constant circuit; said time constant circuit including a capacitor coupled between said keyswitch and said second input lead of said keyer, a first resistor coupled between said second input lead and ground reference to provide a first charging path for said capacitor, and a second charging path coupled to said second input lead and including a gating element coupled to a sustain control terminal; said capacitor and said first and second charging paths having respective values indentical in all of said keying circuits and each of said sets of keying circuits having a separate sustain control terminal; circuit means for applying to said sustain control terminals a plurality of sustain control signals each comprising a chopped voltage signal having a differing preselectable duty cycle for operating said gating elements to produce differing sustain intervals for said sets of keying circuits.
 8. Apparatus as claimed in claim 7, wherein said second charging path further comprises a second resistor and a diode as said gating element coupled between said second input lead and said sustain control terminal. 